This invention is related to the field of chromium catalyst compositions. This invention is also related to the field of ethylene polymerization processes catalyzed with chromium catalyst compositions.
The terms "ethylene polymer", "ethylene polymers" and "polyethylene" for the purposes of this application shall mean homopolymer(s) comprising ethylene and/or copolymer(s) comprising a major amount of ethylene (.gtoreq.50 mole percent) and a minor amount of comonomer &lt;50 mole percent).
It has long been recognized that the processability of an ethylene polymer is significantly affected by the amount of long chain branching in its molecular structure. This is because long chain branches give an ethylene polymer (when compared to ethylene polymers not having long chain branches) a high viscosity under low shear conditions and a low viscosity under high shear conditions. Having a low viscosity under high shear conditions can increase the rate at which the ethylene polymer can be processed, thereby making the process more efficient. Furthermore, long chain branching is known to reduce die swell during blowmolding operations and increase bubble stability during film blowing operations. Since long chain branching has such a large affect on the theological properties, it would be advantageous to be able to control the amount of long chain branching in an ethylene polymer.
Ethylene polymers produced using chromium catalyst compositions have very low levels of long chain branching. However, even at these low levels, long chain branching has a significant affect on the rheological properties of an ethylene polymer. Therefore, it would be advantageous to be able to control the amount of long chain branching in an ethylene polymer that is produced by a chromium catalyst composition.